Electron tube assembly having an incorporated mode suppressor



P. GERLACH 3,381,166 SEMBLY HAVING AN INCORPORATED ODE SUPPRESSOR April30, 1968 ELECTRON TUBE AS M 2 Sheets-Sheet 1 Filed July 9, 1964 HamsieL/wb 009 I WMM. fixlwfl ,ar

April 30, 1968 P. GERLACH 3,381, 66

ELECTRON TUBE ASSEMBLY HAVING AN INCORPORATED MODE SUPPRESSOR Filed July9, 1964 2 Sheets-Sheet 2 United States Patent Oflice 3,381,166 PatentedApr. 30, 1968 3,381,166 ELECTRON TUBE ASSEMBLY HAVING AN INCORPORATEDMODE SUPPRESSOR Pierre Gerlach, Thonon-les-Bains, France, assignor toCompagnie Francaise Thomson Houston-Hotchkiss Brandt, Paris, France, aFrench body corporate Filed July 9, 1964, Ser. No. 381,380 Claimspriority, application France, Aug. 6, 1963, 943,858, Patent 1,373,741Claims. (Cl. 31539) ABSTRACT OF THE DISCLOSURE A VHF-UHF tube havingcoaxial tube elements, and designed for operation in the TEM mode has aTE mode suppressor in the form of an annular Wire of ferromagneticmaterial (e.g., Kovar) located within the tube and between the controlgrid and another element (screen grid, anode), to suppress parasiticoscillations at the location where the TB parasitic oscillations are amaximum, so that optimum suppressing action is obtained.

Electron tubes designed for operation at very high frequencies (thisexpression including the UHF and SHF ranges) are frequently constructedwith coaxial electrodes. These are connected externally of the tube withcoaxial line conductors which, in effect, constitute extensions of theelectrodes and are short-circuited at their outer ends through suitablemeans such as capacitive tuning plungers.

The coaxial electrodes of such a tube and the coaxial lines extendingthem provide resonant circuits capable of oscillating at the desiredhigh frequencies in a desired mode, which typically is the TEM mode.However, other modes of resonance are possible, chiefly the TE mode, andshould resonance in such an unwanted mode be set up in the input circuitof the tube, i.e., between control grid and cathode, and the outputcircuit, as between grid and anode, then a parasitic oscillation at theresonance frequency will be excited, with e.g., two diametricallyopposed halves of the electrode system oscillating in pushpull relation.Such a parasitic oscillation will seriously impair the desired operationof the tube, when used as an amplifier or other device.

It has been usual heretofore to prevent such unwanted secondaryoscillations from being sustained in tubes of the class specified,through the provision of mode suppressor or oscillation dam-per elementspositioned in the coaxial line conductors connected with the tubeelectrodes externally of the tube. The resulting operation, however, hasin many cases been far from satisfactory. The mode suppressor membershad to be placed in empirical fashion, by trial and error, in order toobtain a stronger damping action for the undesirable TE mode than theuseful TEM mode. Frequently the resulting mode suppressing action wasonly effective within a relatively narrow range of operatingfrequencies, and it Was therefore necessary to reposition the suppressorelement, again empirically, every time the operating frequency of thetube circuit was substantially altered.

It is an object of this invention to provide an improved mode suppressorarrangement for coaxial-electrode tubes, which will operate withsuperior efliciency as compared to conventional such arrangements over abroad band of operating frequencies, will not require tedious positionaladjustments on installation and/or in service, and will achieve a highlyeffective suppression of undesired modes, specifically the TB mode,under all circumstances.

The invention provides a coaxial-electrode electron tube assembly havingan incorporated mode suppressor sealed in the envelope of the tube. Themode suppressor is in the general form of an annular element of amaterial having relatively high electric resistivity, disposed ingenerally coaxial relation with the tube electrodes and positionedradially between a pair of the coaxial conductor members provided in thetube (i.e., the electrodes themselves or the coaxial conductors servingto connect the electrodes with the external coaxial conductors), andaxially spaced from the electronically active parts of the electrodes.

One of the reasons why such an arrangement is considerably moreeffective than the conventional arrangements wherein a mode suppressorelement is positioned in the coaxial line conductors outside the tube isthe fact that the strength of the circumferential electric fieldcomponent in a coaxial line oscillating e.g., in quarter-Wave resonance,increases steadily from the shorted to the open end of the line. Hencemaximum efiiciency in suppressing the undesired TE mode is achievablewith the circumferential mode suppressor element positioned within thetube envelope according to the invention.

Exemplary embodiments of the invention will now be described forpurposes of illustration but not of limitation with reference to theaccompanying drawings wherein:

FIG. 1 is a fragmentary view of a coaxial triode in axial elevation withpart of the envelope broken away to show the tube in section, the tubebeing provided with the improved incorporated mode suppressor means ofthe invention;

FIG. 2 is a transverse section of the tube on line AA, FIG. 1;

FIG. 3 is a view similar to FIG. 1 but illustrating a tetrode accordingto the invention; and

FIG. 4 is a section on line B-B of FIG. 3.

The triode illustrated in FIGS. 1 and 2 is of a generally conventionalcoaxial construction Widely used in high-power UHF tubes, and includesan inner cathode 1, a control grid 2 and an external anode 3 which constitutes the envelope of the tube. The cathode 1 is of the directlyheated type and includes an active portion comprising a mesh ofthoriated tungsten wire having its upper end connected by conductiveradial spacer means to an axially extending central conductor rod 8. Thegrid electrode 2 is in the form of a tube closed at its upper end andhaving a perforate wall part adjacent the cathode and spaced radiallyoutward from it. The anode 3 is a massive copper tubular member sealedat its upper end and preferably provided externally with the usualcooling means such as fins, not shown, for dissipating the heatgenerated in operation by means of a stream of cooling fluid in whichthe tube is positioned.

Each of the three coaxial tubular electrodes 1, 2, 3 mentioned above isconnected at its lower end to a related connector member 5, 6, 7respectively, which projects from the lower part of the tube and servesfor the connection of the related electrode with a respective conductorof a coaxial line, not shown. The coaxial connectors 5, 6 and 7 aremaintained in assembly in their prescribed spaced relationship by meansof insulating spacer rings 4, usually made of ceramic. The axial rod 8provides a second connector for the direct-heated cathode 1 and projectsthrough an insulator insert mounted in the centre of the cathodeconnector 5.

In accordance with the invention, an annular element 9 made from amaterial having'relatively high electric resistivity, e.g., Kovar alloy,is supported between the anode 3 and grid 2 coaxially therewith, and isherein positioned in a circumferential recess formed in the lower end ofthe inner anode surface. While the element 9 may be supported in any ofvarious ways in the position shown, in the present example said element9 is shown as being composed of two semi-circumferential parts asclearly apparent in FIG. 2, each part having its ends bent radiallyoutward and secured as by solder to the inner wall surface of anode 3 atdiametrically opposed positions 10 and 11. At points intermediatebetween the areas of attachment 10 and 11 the element 9 is spacedradially inward from said anode wall suface.

In order to understand the significance of the modesuppressor element 9of the invention, the following explanations will be useful.

In the operation of a coaxial-electrode tube of the kind shown in FIGS.1 and 2, the electrode connector members 5, 6 and 7 are connected to theupper ends of respective tubular conductors (not shown) forming coaxiallines and which normally are short-circuited at their lower ends as bymeans of capacitive tuning plugs or plungers. These coaxial lines thusconstitute with the electrodes 1, 2, 3 connected to their upper ends,resonant cavities which normally resonate in the so-called transverseelectromagnetic or TEM mode. In this mode of resonance, the electricfield vectors in any cross section of the coaxial line are all directedin symmetrical relation radially outward or inward and are equal inintensity, while the magnetic lines of force form circumferential linesconcentric with the line conductors. However, such resonant cavitiesalso admit of another mode of resonance, the TB mode, in which theelectric field vectors, while again directed in radial directions, arenonsymmetrical circumferentially, and vary in sense and intensity allaround the circumference of the line cross section so as to constitutean even number of voltage nodes and voltage antinodes around saidcircumference. The associated magnetic force lines are circumferencesconcentrically surrounding the electric field vectors.

The existence of this TE mode of resonance makes it possible forparasitic oscillations to be set up at any frequency higher than thecut-off frequency f for that mode, which corresponds to a cut-offwavelength A substantially equal to the length of the mid-circumferenceof the coaxial line (the circumference equispaced between the inner andouter line conductor) divided by the number 12 of pairs of voltage loopspresent. In most cases, the cut-off frequency f even with 11:1, lies ina frequency range wherein the gain characteristic of the tube is sharplydrooping, so that the setting-up of a parasitic oscillation canpractically occur only at frequency values very close to the cut-offvalue f This condition is usually fulfilled in present-day tubes of thetype specified, because the electrode diameters, and hence the length ofthe midcircumferences of the coaxial lines, are made to approximaterather closely the desired operating wavelengths. Hence the cut-offfrequency f of the coaxial lines connected to such electrodes, in the TEmode, approximates the useful operating frequency.

The reason why the dimensioning of the resonant circuits associated withthe tube increases the likelihood of parasitic TE mode oscillationsbeing set up at frequencies close to the cut-off value f can beexplained by the following considerations.

If it were possible in all cases to tune the coaxial lines forquarter-wave oscillation in the useful TEM mode, with the usefulwavelength being comparable in length to that of the afore-mentionedmidcircumference of the coaxial line, waveguide theory shows that the TEmode resonant frequency would be much higher than f and there would beno danger of parasitic oscillations being excited. However, beyond acertain useful frequency value it is not feasible to use quarter-wavetuning, since this would require the short-circuit tuning plungers beingplaced within the tube envelope. It then becomes neces' sary to applyfii-wavelength and even /4;wavetuning in order to retain acceptablylarge line lengths. As line length increases, the TE mode resonantfrequency asymptotically approaches the cut-off value f and hence variesvery little for the relatively large values of line length obtained withA and wave operation.

Furthermore, since the difference between electrode diameters isrelatively small, the f values for the cathode and anode lines arenearly the same, and hence the natural frequencies of the cathode andanode circuits will be close enough to each other to permitself-excitation due to the feedback coupling inevitably present in thetube as a result of inter-electrode capacitance. Thus the unwantedparasitic TE mode oscillations will quite easily be excited in such atube even through the length of the cathode and anode lines may besubstantially different.

The circumferential mode suppressor element 9 provided according to theinvention within the tube envelope will very effectively prevent such TEmode oscillations from arising. The element 9 provides a path of lowelectric conductivity in the circumferential direction which must befollowed by the conductive current in the TE mode in each cross sectionof the coaxial line if the TE mode of oscillation is to be sustained,and thereby interrupts any incipient parasitic oscillation in that mode.At the same time, the circumferential element does not interfere with,or only opposes a negligibly low resistance to, the conductive flow ofcurrent in the desired TEM mode, since such current flow is effected inthe axial direction.

The element 9, since it is positioned inside the tube, is more effectivethan would be a similar element positioned outside the tube in theexternal coaxial line, since the strength of the circumferential currentcomponent tends to increase continuously from the shorted (lower) to theopen (upper) end of the line, so that the circumferential suppressorelement, when positioned in the tube, exerts a more effective cut-offaction.

According to an important feature of the invention the annularsuppressor element 9 is spaced somewhat from r the conductive walls ofthe electrodes to either side of it as shown, since this causes aconcentration of the electric fields in the region of said elements andthus greatly increases its effectiveness.

The material and the dimensions of the annular suppressor element 9 areselected to provide a total circumferential resistance thereof, foralternating currents at frequencies in the ranges used, e.g., higherthan about 1000 megacycles, which will be at least 10 ohms andpreferably more. According to an advantageous feature of the invention,the element is made from ferromagnetic material. The use of suchmaterial makes it possible to provide the desired over-all A-Cresistance at high frequencies with reasonably large transversedimensions of the element. If non-magnetic materials were used, it wouldbe necessary to reduce the transverse dimensions of the element to suchsmall sizes that the element would be mechanically weak and theresulting tube assembly would not stand up well to shock and vibrationsin service. A satisfactory ferromagnetic material for use in theconstruction of the circumferential mode suppressor element of theinvention is the iron-nickel-cobalt alloy known as Kovar. In onepractical embodiment the ring 9 was 6.3 cm. in inner diameter and wasmade from 2 mm. gauge Kovar wire, and operated extremely successfully insuppressing the parasitic TE mode oscillations in the tube in which itwas mounted.

In the embodiment of FIGS. 3 and 4, the invention is shown applied to atetrode rather than a triode. The tube is constructed essentiallysimilarly to the tube of the first embodiment and corresponding parts ofit have been designated by the same reference numerals. The maindifference lies in the provision of an additional electrode, screen grid12, interposed between control grid 2 and anode 3, and constructedgenerally similarly to the control grid. The screen grid 12 has anassociated annular connector member 13 connected with its lower end andprojecting downwardly from the base of the tube for connection with anexternal coaxial conductor, the connector member 13 being held inposition by means of ceramic spacer rings 4.

An annular mode suppressor element, here designated 14, made fromferromagnetic material, e.g., Kovar wire about 2 mm. thick, is supportedbetween the outer surface of control grid 2 near the base thereof, andthe inner surface of screen grid 12, which is shown as having a radialoffset or shoulder around the element 14. The element 14 may besupported in spaced relation from the surfaces of the grids 2 and 12through any suitable means, including the means shown for supporting themode suppressor 9 in FIGS. 1 and 2. In the present embodiment, however,the mode suppressor element 14 is shown by way of example as being inthe form of a continuous, onepart wire ring and is supported by way of anumber of, e.g., three, radial pins such as 15, having their inner endssoldered to the ring 14 and its outer ends soldered to the inner wallsurface of the electrode 12.

This embodiment operates in a manner exactly similar to that earlierdescribed.

In an alternative, not shown, of the tetrode shown in FIGS. 3-4, themode suppressor 14 may be mounted between the control grid 2 and screengrid 12. High-frequency electron tubes constructed according to theteachings herein disclosed with a built-in mode suppressor operate in amanner greatly superior to that of conventional tubes with external modesuppressors placed in the coaxial-line circuitry connected with the tubeelectrodes. A more effective and reliable dampening of the unwantedoscillation mode is obtained over a wider range of frequencies,eliminating tedious initial adjustments on installation andre-adjustments in service.

It will be apparent that various modifications may be introduced anddepartures made from the embodiments shown and described withoutexceeding the scope of the invention. Thus, the whole circumference ofthe mode suppressor may be in actual contact with the inner or outercylindrical surface of an electrode of the tube. Or else, more than onemode-suppressor-rings of the kind here specified might be provided, suchas one between the cathode and control grid, and the other between thescreen grid and anode, of a tetrode.

I claim:

1. An electron tube assembly having a number of coaxial, annularconductive electrode elements and electrode-connector elementspositioned within the tube and connectible exteriorly of the tube withrespective coaxial conductors constituting respective resonant circuitstherewith, and at least one annular member of a conductive, relativelyhigh resistance material, said member being supported within the tubebetween a pair of said conductive elements coaxially therewith toprovide a circumferential path of relatively high electrical resistanceand damping out unwanted TE mode oscillation in said resonant circuits.

2. A tube assembly according to claim 1, wherein said annular member isconstructed to provide an over-all electric resistance of at least tenohms around its circumference for alternating currents at substantiallythe operating frequency range of said tube.

3. A tube assembly according to claim 1, wherein said annular member ismade from a ferromagnetic material.

4. An electron tube assembly having a plurality of coaxial, annularconductive electrode elements therein; coaxial, annular conductiveelectrode connector elements having their one ends connected to ends ofsaid electrode elements and having opposite ends connectable withrespective coaxial conductors exteriorly of the tube to providerespective coaxial resonant circuits, and an annular member means madefrom ferromagnetic material and supported within the tube betweencircumferential wall surfaces of a pair of adjacent ones of saidconductive elements coaxially therewith to provide a circumferentialpath of relatively high A-C resistance in at least one of said resonantcircuits near the upper end thereof to damp out unwanted TE modeoscillation in said resonant circuits.

5. An electron tube assembly having a plurality of coaxial, annularconductive members therein, means for connecting outer ends of saidmembers exteriorly of the tube with respective coaxial conductors, atleast one annular shaped mode-suppressor element made from ferromagneticmaterial positioned within the tube between circumferential surfaces ofa pair of adjacent ones of said conductive members, and means supportingsaid modesuppressor element in radially spaced relation from saidsurfaces.

6. The tube assembly defined in claim 5, wherein said annularmode-suppressor element comprises at least two part-circumferential wiresections having radially bent extremities connected with one of saidcircumferential surfaces, said part-circumferential sectionssubstantially abutting at the bend point of the extremities toconstitute a substantially complete circumference.

7. The tube assembly defined in claim 5, wherein said annularmode-suppressor element comprises a ring of ferromagnetic wire andradial pins are provided for connecting spaced points of said ring withone of said wall surfaces.

8. The tube assembly defined in claim 5, which comprises a cathode, agrid and an anode electrode members, and said annular mode-suppressorelement is positioned between said grid and anode members.

9. The tube assembly defined in claim 5, which comprises a cathode, acontrol grid, a screen grid and an anode electrode members, and saidannular mode-suppressor element is positioned between said screen gridand anode members.

10. The tube assembly defined in claim 4, wherein said annular member issupported between circumferential surfaces of said electrode connectorelements at locations nearer the base than the top of the tube.

References Cited UNITED STATES PATENTS 2,502,456 4/1950 Hansen et al333-98 X 2,806,951 9/1957 Willwacher et al. 33056 X 2,867,726 1/1959Preist 33056 X 3,209,276 9/1965 Fricke et a1 33383 HERMAN KARL SAALBACH,Primary Examiner. ELI LIEBERMAN, Examiner.

S. CHATMON, JR., Assistant Examiner.

